Premixing apparatus

A premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan,

• wherein a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the air supply passage, which is located on an upstream side from the gas suction portion, and
• wherein, provided that a rotational axis line along a predetermined radial direction of the butterfly valve serves as a center, the butterfly valve is configured to be rotatable to at least two attitudes:
• an open attitude parallel to a longitudinal direction of the air supply passage and a closed attitude orthogonal to the longitudinal direction of the air supply passage,
• wherein:
• at least one ventilation opening through which air flows from an upstream side to a downstream side of the butterfly valve at the closed attitude is formed in the butterfly valve, and a size of the butterfly valve is set so that, at the closed attitude, a clearance between an outer peripheral edge of the butterfly valve and a peripheral wall surface of the air supply passage is substantially closed, and air flows only through the ventilation opening.